Number needed to treat calculator
NNT is 1 divided by the control event rate minus the treated event rate. A negative difference is reported as number needed to harm.
Relative risk for the same table is on the relative risk calculator.
Questions
What is number needed to treat?
Number needed to treat is how many people you would treat, instead of giving the control, for one extra person to avoid the event. It is 1 divided by the absolute risk reduction.
What is number needed to harm?
If the treated group has more events than the control group, the same calculation is number needed to harm: how many people treated for one extra event.
Why is my number needed to treat so large?
A small absolute difference makes a large NNT. A relative risk that looks impressive can still have a large NNT when the control event rate is low.
Educational only. Not medical advice, not a regulatory determination, and not a substitute for qualified RA/QA, legal, clinical, or statistical counsel. Number needed to treat does not submit to FDA, Notified Bodies, or IRBs.